The Limit Between Ectoderm and Entoderm. 57 
sagittal and transverse sections it is seen that this layer turns in 
around the entoderm (Figs. 8, 9, 10, 12, 13). This in-growing 
ectoderm pushes in farther on the dorsal surface of the entoderm than 
on the ventral and soon forms a ring or collar around the solid oral 
entoderm. At first the ring is incomplete in its mid-dorsal portion 
where the inner layer of ectoderm is taken up in the formation of 
the hypophysis. The ingrowing ectoderm spreads to fill up this 
Fic. 15. A. punctatum, formation of dental ridges. A transverse section 
through the mouth plate after disappearance of the ectoderm. Hzemalum. 
gap, and pushes up on the dorsal surface of the oral entoderm to 
about the point of contact of preoral entoderm and hypophysis. 
This mid-dorsal portion of the in-growing ectoderm gives rise to the 
vomerine teeth. The in-growing ectoderm forms the dental ridges 
in which the teeth soon begin to make their appearance (Figs. 12- 
18). For the purpose of this study the formation of the teeth need 
not be followed in detail. One point only is of especial interest 
here, namely, that when the teeth are formed they are separated from 
